Light valve device with failure detection circuit

Abstract: Pixels, which include both thin-film type transistors (1) and liquid crystal cells (3) driven by liquid crystal driving electrodes, are arranged in a matrix shape together with image signal lines (4) and control signal lines (5) connected to each of the transistors. A control signal line driving circuit (6) is formed of shift registers having one bit per signal line, and data signals inputted on scanning-start, synchronised with the clock signal. The driving circuit outputs the signals capable of turning-ON the thin-film transistor of the corresponding control signal line from a shift register on a position which is moved by one bit per every clock cycle. A detecting circuit (9) of the image signal line driving circuit (8) have three terminals, the first terminals are connected to the image signal lines (4), the second terminals are connected to output lines (10), and the third terminals are connected to outputs (11) of the shift registers, so that failures of the driving circuits can be detected simply.

Description: The present invention relates to inspection circuits of flat type light valve devices used for direct visual type display devices or projection type display devices. More specifically, it relates to light valve devices, for example, inspection circuits of active-matrix liquid crystal display devices, which incorporate integrated circuits as a liquid crystal panel formed of driving circuits unitarily into semiconductor thin films. An active-matrix type liquid crystal display device operates by using an extremely simplified principle. Switching elements are provided on each pixel. When selecting a specified pixel the corresponding switching elements are conducted, and when non-selecting, the switching elements are put into a non-conductive state. The switching elements are formed on a glass substrate constituting a liquid crystal panel. It is therefore important to realise the technique for more satisfactorily producing thin-film shaped switching elements. For such elements, thin-film type transistors are generally used. The conventional active-matrix device, as is shown in a schematic circuit diagram in figure 6, comprises; pixels each arranged in a matrix shape in vertical and horizontal directions which are formed of thin-film transistors 1 and electro-optic elements 3 such as liquid crystal; control signal lines 5 provided on gate electrodes of the thin film transistors 1; image signal lines 4 connected to source electrodes; an image signal line driving circuit 8 connected to the image signal lines 4; and an image signal line driving circuit 6 connected to control signal lines 5. The control signal line driving circuit 6 is mainly formed of shift registers, where each unit-bit output is connected to the signal lines 5. The image signal line driving circuit 8 is formed of shift registers and sample hold circuits provided at every bit basis, and writes the image signals into the sample hold circuits in accordance with sampling signals from the output of the shift registers. The conventional light valve device has more than several hundreds pixels respectively for vertical and horizontal directions. The entire pixels thus reach the number of one million and generally over at least an area of more than 1 cm 2 . It is considerably difficult to produce such elements with a high production yield without any defects. In general the produced elements are inspected in a form of the driving substrate before completion as a light valve device. The most normal method of inspection, to determine acceptability or failure, includes measuring the current produced by applying a voltage through a metallic probe (hereinafter referred to as a prober) which is in touch with the electrodes of the elements, or for the output voltage/current etc. In the method described above, to confirm operation of the elements formed of a large number of pixels of the light valve devices or the like, the number of probers required to contact the electrodes of elements at an interval corresponding to a pitch between pixels is more than several hundreds. It is therefore difficult to obtain a reliable result in the present technique. On the other hand, the measurement while using only a small number of probers produces a long time for the measurement and is not suitable for practical use. Another method of inspection has been considered and provides inspection circuits inside the elements. Figure 6 shows an equivalent circuit diagram of the elements used in such an inspection method. Transistors 23 having the gate electrodes connected to the signal lines 4 are provided on signal output sections ranging from each driving circuit to the pixels. In the inspection transistors 23, one terminal 24 is grounded and the terminals on the other side are connected to common terminals 25. Thereafter, the common terminals are connected to a power supply 27 through a load resistance 26. An output of the load is then detected by the inspection transistors 23 at every bit. Signals from the driving circuit are applied to the signal lines 5 to turn ON the inspection transistors 23 and to produce current flow into the load 26, thus with such current flow detected, the signal transfer to the signal lines 5 are confirmed. By observing timing of current flow in synchronism with the clock of the shift register, a bit relating to the operation can be determined to thereby detect a line on which the malfunction arises. However, in the inspection circuit of the light valve device signals are detected in an output of a buffer amplifier. If only one of the detecting FET's having several hundreds bits is turned-ON, the inspection circuit of the light valve device does not determine on which of the bits the defect is generated in case of the driving method simultaneously originating signals for a plurality of bits. The image signal driving circuit generally produces the outputs at the same time from the entire lines. Document EP-A-480819 discloses a circuit for controlling the lines of a display including means for the testing of sampler-holder circuits. Document WO-92/11560 discloses an active control matrix electro-optical screen comprising a built-in test system. Document EP-A-143039 discloses a process for producing a matrix of electronic components, a row or column of which is tested by means of optical addressing. The present invention, provides a function to control the detecting operation on the basis of every bit. Thus, the present invention securely performs the detecting operation only at specified bits to exactly find a cause of the defect. With the malfunction securely determined, the failed components or parts are removed in the form of a driving substrate. At the same time the cause of the malfunction is fed back and thus reduces generation of such malfunctions again. The present invention uses an electrical method, and this enables rapid measurement. According to the present invention there is provided a light valve device having: a driving substrate having a plurality of control signal lines and a plurality of image signal lines for providing image display signals; a counter substrate; an electro-optic material arranged between the driving substrate and the counter substrate; a plurality of pixels each of which comprises, a switching element, a driving electrode connected electrically to the switching element, and the electro-optical material in a respective intersection of the control signal lines and the image signal lines; a driving circuit for driving the control signal lines and the image signal lines; and a confirmation circuit comprising a three terminal switching element and an inspection signal output line; characterised in that a first terminal of said three terminal switching element is connected electrically to one of the control signal lines and the image signal lines; a second terminal of said three terminal switching elements is connected electrically to the inspection signal output line; and a third terminal of each of the three terminal switching elements is connected electrically to the output of a respective shift register of the driving circuit and serves for controlling electrically the turning ON and OFF of the three terminal switching element disposed between the first terminal and the second terminal for inspection of the driving circuit. Switching elements capable of performing connection/disconnection of input from signal lines to detectors are provided to detect signal levels of the signal lines during input or after completion of the input. In addition, the timing of the signal potential detection of the signal lines are controlled to detect independently each operation of the entire signal lines. Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 shows one embodiment of an inspection circuit of the present invention; Figure 2 shows another embodiment of an inspection circuit of the invention; Figure 3 shows another embodiment of an inspection circuit of the invention; Figure 4 shows one embodiment of a circuit of detecting section of a inspection circuit of the invention. Figure 5 shows another embodiment of an inspection circuit of the invention; and Figure 6 is a circuit example of the conventional active-matrix type liquid crystal display panel. Figure 1 is a circuit diagram showing an embodiment of the present invention. In the drawing, the pixels include both switching elements 1, made of thin-film type transistors, and liquid crystal cells 3, being electro-optic material driven by liquid crystal driving electrodes 2 connected to the drain electrodes of the thin-film type transistors. The pixels are arranged in a matrix shape having rows and columns. One image signal line 4 is connected to a source of each pixel transistor on one column, and one control signal line 5 is connected to a gate electrode of each pixel transistor on one row. A control signal line driving circuit 6 is formed of shift registers having one bit per signal line. Data signals are inputted into a data input line 62 on scanning-start, these signals are synchronised with a clock signal of a control signal clock input line 61. The control signal line driving circuit outputs the signals capable of turning ON the gate of the thin-film transistor 3 to the corresponding control signal line 5 from a shift register whose position is moved by one bit per every clock cycle. An image signal line driving circuit 8 is formed of shift registers 81 having the bit number corresponding to the number of columns of pixels, and sample hold circuits 82 connected to the shift registers of each bit. As in the control signal line driving circuit, outputs from the shift registers feed image-signal sampling signals to the sample hold circuit 82 while moving by one bit per every clock of clock signals on a clock signal input line 84. Thus image signals from an image signal input line 83 are held in the sample hold circuits. Outputs of the sample hold circuits are output to the image signal lines 4 through amplifiers etc. Detecting circuits 9 are provided each having three terminals, where the first terminals are connected to the image signal lines 4, the second terminals are connected to output lines 10, and the third terminals are connected to outputs 11 of the shift registers. When the output of the shift registers is high "H", synchronously thereto the image signal come to an ON-state, and feed signals now being applied to the image signal lines 4 to the output line 10. That is, the output of the inspection signal output line 10, only when a shift register of a specified bit is "H", detects and outputs the image output corresponding to such bit. In the shift registers, only one bit outputs "H" at any one time. Thus even when inputs from a plurality of bits exist in parallel with each other in the output buffer, then only the image output of a specified bit can be detected in specified timing. This therefore results in detecting each image output of a plurality of bits independently. Figure 2 shows another embodiment of a detecting circuit according to the present invention. In this embodiment an input of a detecting circuit 12 which is coupled to a detecting control signal terminal 13 differs from figure 1 in that the signals of the input terminals 11 and 13 pass through a logic product circuit. Thereafter, in accordance with the logic product value, it is determined whether or not an output to the terminal 10 is performed. When a detecting control signal is "L", the control proceeds in that no detecting is performed: even when the output signals of the adjacent bits of the shift register overlapped timing, then the output from the specified bit can be detected by designating the detecting timing using the detecting control signals. Otherwise, any timing overlap with the adjacent bit is prevented by adding the shift-register inverted signal of an adjacent bit to the detecting control signal. Figure 3 shows a detecting circuit of a light valve device showing another embodiment of the present invention. In figure 3, the signal detecting circuit 12 and a signal detecting circuit 15 are provided on both ends of the image region of the image signal line 4 respectively. A driving circuit 16 for scanning the second detecting circuit 15 is also provided independently from the first driving circuit 8. In the first and second driving circuits, two methods are employed, namely, one in which the shift clocks thereof are synchronised, or in which respective independent shift clocks are used. The detecting circuits 12 and 15 provided on both ends of the signal line are able to detect a signal line defect such as disconnection of the signal line etc. Specifically, when signals are detected by the first detecting elements and not detected by the second detecting elements, disconnection is determined to exist intermediate of the signal line. The detecting circuits can readily be formed of a transmission gate 17 and an amplifier 18 and the like as shown in figure 4. An output 13 and a detecting control signal 14 of the shift register are fed through a logic product circuit 19 to be input into the transmission gate. Figure 5 shows another embodiment of the present invention, where an inspection circuit is also provided on both sides of the control signal lines. The control lines 5 are connected to detecting signal input terminals of detecting circuits 20 and 20'. The connections of inspection output terminals 21 and 21' and signal detecting control terminals 22 and 22' are similar to the detecting circuits 12 and 15. According to the present invention, in observing an inspection signal output there can be made independently for all the signal lines a decision as to whether it is "normal" by detecting the output signals to images at a specified timing. Otherwise, the decision is that there is a "malfunction" if it is impossible to detect the same. In addition, the inspection circuit is incorporated in the element to enable inspection without using the prober etc. When the detecting circuit is provided on both ends of the signal line, either of the driving circuit or the image region is determined as a position where a malfunction arises. This improves production yield in coping with the cause of the malfunction in the production process. The inspection can be performed during the time corresponding to the display of one image picture, which is possibly within several tens of milli-seconds. Furthermore, the detecting circuit connected to the image signal output line, if using an analog input/output, determines whether or not the suitable image picture signal value is being obtained as an analog value, in addition to whether or not the signal is present. Moreover, if the control signal line is linked with the image picture signal line, it is determined whether it is satisfactory or not at every pixel basis. After writing image signals into the pixels, signals within the pixels are output to the image picture signal line, as in a DRAM, to detect and amplify the thus produced output by the detecting circuit. Thereby it is determined whether or not the image picture signal is written and held in the pixels. As hereinbefore fully described, according to the present invention, a circuit is disclosed for detecting any malfunction in operation which may be unitarily formed inside the elements, failures of driving circuits and malfunctions in pixels together with the positions of such malfunctions and failures. The circuit also achieves a compact size display device having the driving circuit formed on the same substrate. Further, a remarkable effect is obtained in considerably reducing the measurement time. The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.

Claims: A light valve device having: a driving substrate having a plurality of control signal lines (5) and a plurality of image signal lines (4) for providing image display signals; a counter substrate; an electro-optic material arranged between the driving substrate and the counter substrate; a plurality of pixels each of which comprises, a switching element (1), a driving electrode connected electrically to the switching element, and the electro-optical material in a respective intersection of the control signal lines and the image signal lines; a driving circuit (6, 8) for driving the control signal lines and the image signal lines; and a confirmation circuit (9) comprising a three terminal switching element and an inspection signal output line (10); characterised in that a first terminal of said three terminal switching element is connected electrically to one of the control signal lines (5) and the image signal lines (4); a second terminal of said three terminal switching elements is connected electrically to the inspection signal output line (10); and a third terminal of each of the three terminal switching elements is connected electrically to the output of a respective shift register of the driving circuit and serves for controlling electrically the turning ON and OFF of the three terminal switching element disposed between the first terminal and the second terminal for inspection of the driving circuit. A light valve device as claimed in claim 1, wherein the third terminal of said three terminal switching element is also connected electrically with the output of a logic product of other external input signals. A light valve device as claimed in claim 1 or claim 2, wherein the shift registers are synchronised with a clock pulse. A light valve device as claimed in any preceding claim, wherein the plurality of image signal lines and the plurality of control signal lines define an image region, and the driving circuit and the confirmation circuit are formed around the image region on the driving substrate, the confirmation circuit being provided on one side and on the other side of the image region.

Inventors: SAKURAI ATSUSHI, TAKAHASHI KUNIHIRO, TAKASU HIROAKI, YAMAZAKI TSUNEO, SAKURAI, ATSUSHI, TAKAHASHI, KUNIHIRO, TAKASU, HIROAKI, YAMAZAKI, TSUNEO, SAKURAI, ATSUSHI, C/O SEIKO INSTRUMENTS INC., TAKAHASHI, KUNIHIRO, C/O SEIKO INSTRUMENTS INC., TAKASU, HIROAKI, C/O SEIKO INSTRUMENTS INC., YAMAZAKI, TSUNEO, C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1994
Main classification: G06F11
IPC codes: G09G3, G02F1